Field of the Invention
The invention concerns a method to acquire at least two measurement data sets of an examination subject to be examined by means of a magnetic resonance system, as well as a magnetic resonance system; and an electronically readable data medium for implementing such a method.
Description of the Prior Art
Magnetic resonance (MR) is a known technique with which images of the inside of an examination subject can be generated. Expressed in a simplified form, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (also called a B0 field) with a field strength from 0.2 Tesla to 7 Tesla or more in a magnetic resonance apparatus, such that the nuclear spins of the examination subject orient along the basic magnetic field. To trigger nuclear magnetic resonance signals, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, and the triggered nuclear magnetic resonance signals are measured (ducted) as what are known as k-space data, on the basis of which MR images are reconstructed or spectroscopy data are determined. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with values, for example by means of a multidimensional Fourier transformation.
Magnetic resonance examinations are most often very loud. The main reason for this is the rapidly changing gradient fields, which lead to distortions and oscillation in the gradient coil, and this energy is transferred to the housing, and is perceived as banging by patients located in a magnetic resonance system. In addition to mechanical measures for noise reduction, it can also be sought to design the measurement sequences used for the examination to be as quiet as possible by keeping changes of the required gradient fields over time (dG/dt) as small as possible.
In general, most measurement sequences acquire measurement data by repeating a basic scheme (which includes at least one excitation pulse, gradients for spatial coding and a readout module) with differently switched gradients after a repetition time TR. An implementation of such a basic scheme thus corresponds to a partial measurement (also called a repetition).
MR measurement (data acquisition) sequences that are designated as “quiet” are known in which the gradients to be switched do not change, or change only insignificantly, during such a partial measurement and between successive repetitions. Examples of such measurement sequences are, for example, RASP (“Rapid Single Point”), SPRITE (“Single-Point Ramped Imaging with T1 Enhancement”), zTE (“Zero Echo Time”), SWIFT (“Sweep Imaging with Fourier Transformation”) and PETRA (“Pointwise Encoding Time reduction with Radial Acquisition”).
Furthermore, in MR examinations it is typical to implement a measurement multiple times and to average the different measurements in order to increase the signal-to-noise ratio (SNR) of the measurement. Two possibilities of data acquisition are known for MR measurements with at least one such averaging. A first possibility is to implement a measurement completely and, at the end thereof, the same measurement is started again in its entirety and repeated as often as desired (MDS1-MDS2-MDS3 . . . ). This procedure is also designated as an “outer averages” method. A second possibility is to repeat each partial measurement (repetition) as often as desired before the next partial measurement is started (again as often as desired) (MDS1_1-MDS2_1-MDS3_1- . . . -MDS1_2-MDS2_2-MDS3_2- . . . -MDS1_3-MDS2_3-MDS3_3- . . . ). This procedure is also designated as an “inner averages” method.
In the aforementioned quiet measurement sequences, however, the “inner averages” cannot be applied, because otherwise unwanted refocusings of residual magnetization between two identical partial measurements (MDSj_i-MDSj+1_i) lead to artifacts in an MR image reconstructed from the measurement data. However, the “outer averages” method has the disadvantage that it is very susceptible to patient movements, since partial measurements to be averaged with one another are acquired with a relatively large time interval.